Corrugated horn antenna

ABSTRACT

A horn type of antenna for the propagation of electromagnetic energy having a modified structure for the reduction of backlobes and sidelobes in the radiated beam by controlling the illumination of the E-plane edges. Specifically, the control of illumination of the E-plane edges is achieved by electrically modifying the walls of the horn having an E-plane edge as an element.

oat-a1 U United States Patent Inventors Appl. No. Filed PatentedAssignee Leon Peters, Jr.;

Richard E. Lawrie, both of Columbus, Ohio 500,024

Oct. 21, 1965 Dec. 28, 1971 The Ohio State University ResearchFoundation The portion of the term of the patent subsequent to July 28,1987, has been disclaimed.

CORRUGATED HORN ANTENNA 5 Claims, 10 Drawing Figs.

US. Cl 343/786 Int. Cl H011 13/00 Field of Search 343/786, 840

References Cited UNITED STATES PATENTS Cutler Ashbaugh et a1. Schusteret a1.

Kay Kay Lewis Primary Examiner- Eli Lieberman AttorneyAnthony D. CennamoABSTRACT: A horn type of antenna for the propagation of electromagneticenergy having a modified structure for the reduction of backlobes andsidelobes in the radiated beam by controlling the illumination of theE-plane edges. Specifically, the control of illumination of the E-planeedges is achieved by electrically modifying the walls of the horn havingan E-plane edge as an element.

PATENTEDBEB28|97| 3.631.502

SHEET 1. BF 5 8 E EDGE 8==HORN ANGLE INVENTOR 3 LEON PETERS JR.

RICHARD E. LAWRIE BY %j %&MM 1-" ATTORNEY PATENIEmmen 3.631; 502

SHEET 3 OF 5 l5 CONTROL HORN SMALL CORRUGATED HORN i i so I20 FIG.6

INVENTOR LEON PETERSJR. RICHARD LAWPIE ATTORNEY PATENTEU DEC28 :sn

SHEET 4 0F 5 INVENTORS LEON PETERS,JR. RICHARD E. LAWRIE mm ATTORNEYPATENTEDHEBZBIQYI 3.631.502

SHEET 5 OF 5 FIG. 9

IN OR. LEON PETERS JR. BYRICHARD E. LAWRIE CORRUGATED HORN ANTENNABACKGROUND OF THE INVENTION It is well known that most antennas havesidelobe structures which can create interference. The general problemhas been reported and the sources of these sidelobes have also beendescribed.

In a pyramidal horn antenna the electric field vector is perpendicularto one pair of aperture edges, designated as E- plane edges. It has beenshown that most of the backlobe structure of a horn is due to energydiffracted by a the E-plane edges of the horn. In fact, the entireE-plane pattern of a particular horn has been accurately calculated bytreating the diffraction from such edges as well as the geometricaloptics field.

SUMMARY The present invention is directed to the elimination of thesidelobes and backlobes by controlling the illumination of the E-planeedges. That is, the energy is prevented from illuminating the edges fromwhich it is diffracted into the back regions.

Specifically, the control of illumination of the E-plane edges, toeliminate the undesirable backlobes from a radiation field, is achievedby electrically modifying the walls of the horn having an E-plane edgeas an element.

OBJECTS OF THE INVENTION It is accordingly a primary object of thepresent invention to control the energy distribution in a radiated beam.

Another object of the present invention is to reduce the sidelobes andbacklobes in the radiated beam of a horn anten- A further object of thepresent invention is to electrically modify the walls of the antennahaving an E-plane edge.

Still another object of the invention is to provide means foreliminating sidelobes and backlobes from a radiated beam and which meansare readily adaptable to existing structures.

Further objects and features of the present invention will becomeapparent from the following 'detailed description when taken inconjunction with the drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic illustration ofthe edge diffraction from a pyramidal horn antenna;

FIG. 2 illustrates the surface of a circular horn modified to include anarray of bent choke slots;

FIG. 3 illustrates the surface of a horn modified in a preferredembodiment to include a corrugated surface;

FIG. 4 3 db. beamwidth of the small corrugated horn and the control hornas a function of frequency;

FIG. 5 ratio of backlobe to main lobe level of the small corrugated hornand the control horn as a function of frequency;

FIG. 6 E-plane patterns of the small corrugated horn and the controlhorn measured at gc.;

FIG. 7 E-plane patterns of the large control horn and the largecorrugated horn measured at 10 gc.

FIG. 8 is a side view schematic illustration of a horn utilizing thecorrugated surface of the present invention;

FIG. 9 is an end view -looking into the throat of a constructedembodiment of the invention; and

FIG. 10 is a perspective view of a constructed embodiment of a hornantenna utilizing the corrugated surface of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 there isillustrated the edge diffraction from a pyramidal horn antenna. Theportion of the energy directly radiated from the horn throat and notdiffracted by the sides of the structure results in the desiredradiation pattern. Radiation due to energy incident upon the E-planeedges is diffracted into space as shown in FIG. 1. No significantsimilar diffraction occurs at the I-I-plane edges for the angles ofincidence involved. The diffracted energy from the E-plane edges resultsin the unwanted backlobes and sidelobes.

Illumination of the E-plane edges is prevented by electrically modifyingthe walls of the horn having an E-plane edge as an element. Themodification may be achieved by the of three methods: by lining thewalls 12 with a microwave absorber; by a series of choke slots, as shownin 10a xxx l0n of FIG. 2, cut into the walls 12; or by creating areactive surface at the walls 13, as shown in 15a xxx 15a of FIG. 3.

The analysis of the preferred embodiment of an infinite corrugatedsurface, shown in FIG. 3, may be considerably simplified by making thefollowing assumptions:

1. The slot walls (teeth) are vanishingly thin;

2. Only the TEM mode in the slots is reflected from the base of theslots. The higher order modes are attenuated before reaching the base.

The second assumption is equivalent to requiring that the slot width (g)be small compared with both the free-space wavelength and the slot depth(d). For such a surface, it is shown that the reactance of the surfaceis given to a good approximation by 1: 10,01 x g t 6 an provided thatwherein g slot width t= thickness of a tooth p.= permeability ofmaterial used E dielectric constant of material used k,,= propagationfactor d depth of slot This condition is satisfied if tg/lO and thesecond assumption is valid for g 10 The surface reactance must becapacitive so that the surface will not support a surface wave, or, fromEq. (1) J4 d )./2. Of course d may be within some odd integral multipleof this interval. It has been shown that the cutoff depth d depends tosome extend on the slot width (3). However, curves previously derivedindicate that for g 4-)! l0 the cutoff region is approximately )t/4 dA/2.

Microwave-absorbing material applied to the walls of a horn produced asignificant reduction in the backlobe level. Unfortunately, it alsoproduced a serious reduction in the overall gain of the antenna.

The absorber lined horn comprised an X-band, pyramidal horn, 13% inchhigh with a 9-inch 9-inch aperture. A /4-inch thick absorber was placedalong the walls of the horn so that it protrudes slightly beyond theE-plane edges. The resultant pattern resulted in a 13 db. change in thebeam maximum from energy loss in the absorber. Smoothing of the mainbeam and reduction of the backlobes with respect to the main beam wasachieved as the result of the elimination of E-plane edge illumination.In any communication system, the severe loss in gain due to energy lossin the absorber would completely nullify the improvement in backlobe andsidelobe levels.

It has been shown that choke slots cut into the walls of a horn cansignificantly reduce the back lobe level. This arrangement satisfies theabove two assumptions and yields a significant reduction in backlobelevels without a loss of overall gain but only over a narrow frequencygain.

A prior art horn antenna of FIG. 2 is a circular horn with the type ofbent choke slot modified as shown in FIG. 2. The choke slots areseparated by nearly half a wavelength and thus do not meet the conditionof Eq. (1). Nevertheless the results obtained with this modificationwere impressive and the backlobes reported are approximately 34 db.below the pattern maximum at .the design frequency. This feed is not ascffective over as broad a frequency band as the preferred embodiment ofFIG. 3, i.e., the corrugated horn and the backlobes are an order of amagnitude higher.

A small choked horn was constructed with a designed aperture 3% inchsquare and with 92 flare angle. The E-plane walls are five-eighths inchthick with four evenly spaced choke slots per wavelength. The chokeslots are designed to be threeeighths wavelength deep at gc. Acontrolling parameter for the choked horn was obtained by placing stripsof aluminum tape over the chokes and painting over the chokes withsilver paint. This leaves a horn with smooth thick E-plane walls whoseeffect on the antenna pattern has already been investigated.

The backlobe level of the choked horn is about 38 db. at the designfrequency of 6.6 gc. where the choke depth is onequarter wavelength butincreases exponentially as the frequency increases. The backlobe levelof the choked horn was 3 db. below that of the control horn at 12 gc.whereas it is 12 db. below the control horn at the design frequency of6.6 gc. The performance of the choked horn is only slightly improvedfrom that of the prior art feed.

In a first preferred embodiment a relatively small corrugated horn wasconstructed in accordance with FIG. 3 with 3'1-inch square aperturehaving an internal E-plane wall structure of many slots per wavelength(about four times that of the choked horn, or 15 per wavelength at 10kmc. This new horn was designed with a flare angle of 50 to compare withsmall horn patterns previously assembled.

The conventional control horn for this structure was again that when theslots were covered with aluminum tape and paint. The curves of backlobelevel vs. frequency for both antennas are shown in FIG. 5. From this adefinite passband within which the corrugated walls have their greatesteffectiveness can be seen from 8 to 14 gc. The average backlobe is foundat 42 db. below the main beam and is nearly independent of frequency inthis band, while the beamwidth at these frequencies ranges from 16 to22. The backlobe level of the control horn for these same frequenciesvaries, with an average of 32 db. or 10 db. above the corrugated horn.

FIG. 4 shows the 3 db. beam width-vs.-frequency curves of the corrugatedhorn and the control horn. The two are similar but displaced by about42. In contrast with the choked horn, the corrugations have resulted inan increase in beamwidth.

The E-plane pattern of the small corrugated horn superimposed on theE-plane pattern of the control horn is shown in FIG. 6. Both patternswere obtained at 10 gc. It is also significant that the pattern of thesmall corrugated horn is free of the more rapid variations seen in thepattern of the control horn. These undesirable rapid variations aretypical of horn antennas.

In a second preferred embodiment a relatively large corrugated horn wasconstructed and tested. The horn shown in the several views of FIGS. 8,9 and 10 comprised a thick-walled pyramidal horn having a flare angle of34, a height of 5.85 inches, and a 9.7 inch aperture. The corrugatedsurface is machined into the proper walls from within 1 inch of thethroat to the mouth. The large control horn is a thick-edged horn havingthe same interior dimensions. The E-plane pattern of the large controlhorn is shown in FIG. 7. The I'l-plane patterns of the large corrugatedhorn are nearly identical to that of the large control horn and are notshown. These patterns were obtained at 10 gc.

The approximate directivity of the large control horn is 21 a db. andhence the energy that would yield higher backlobes and hence causeinterference has been converted to useful energy in main beam. Thechange in directivity is caused by the reduction of the E-planebeamwidth and by the removal of the saddle in the E-plane pattern of thecontrol horn. The saddle is attributed to edge effects which are removedby the action of the corrugated surface.

The backlobe of the large corrugated horn is 57 db. below the main beam.Thus it is 27 db. better than the difference between the main andbacklobes of the control horn.

Initial measurements of the E-plane pattern of the large corrugated hornindicated severe interference effects throughout the back hemisphere. Itwas found that the primary source of the interfering signals was leakagethrough the waveguide joints and components (i.e., detector,attenuator). A rearrangement of components and the judicious applicationof aluminum tape and metallic paint greatly reduced the interference.The remaining interference is attributed to scattering from variousstructures in the vicinity of the pattern range.

The use of a cutoff corrugated structure in the walls of a horn antennahave been demonstrated to be effective methods of reducing the backlobelevel of the horn. The use of corrugated surfaces produces a greaterimprovement than the choke slots and results in an increase inbeamwldth, gain and a bandwidth. The attainable reduction in backlobelevel is limited by diffraction from the wedge formed by the waveguideand the wall of the horn to the edge of the opposite wall. It was foundthat the usable bandwidth of the modified horns is at least as great asthe bandwidth of the transmission line feeding the horn.

The type of modified horns described above may find applications, suchas use in pattern ranges and radar cross section ranges. The applicationof this type of antenna as a feed will result in a good low-temperatureperformance required in many modern systems, and will also be useful inthe reduction of interference between various systems. Further uses forcutoff corrugated surfaces may be in screening fences for the reductionof interference and ground clutter in radar systems. Furthermore, cutoffcorrugated surfaces might find application in the isolation of anantenna from surrounding surfaces, such as an air frame; however, suchapplications require further study.

With reference to FIG. 8 there is shown schematically an antenna hornhaving the above-described corrugated surfaces. In FIG. 9 there is apictorial illustration-looking into the throat of the antenna horn-ofthe corrugated surface of the present invention. Also in FIG. 10 thereis a pictorial illustration of the antenna horn utilizing the corrugatedsurface of the invention.

Although in the embodiments shown in FIGS. 8, 9, and 10, the corrugatedsurfaces are in the upper and lower panels (E plane), it is to beunderstood as set forth above that the invention is equally applicablewith the corrugated surfaces in the side panels (l-I plane).

Other modifications to the embodiments shown are within the scope of theinvention.

We claim:

I. An antenna including a pyramidal horn structure having a flare angleless than for directing the electromagnetic energy in a given directionand wherein energy is diffracted from certain of the surfaces of saidstructure, the improvement comprising means for controlling theillumination of said surfaces, said means including corrugated slotsformed in said surface perpendicular to the transverse E vector and theiris of said horn; wherein the depth of said slots is defined by k h r dr and the slot width mid spacing is defined by with t much less than g,where g is the slot width and t is the wall thickness of thecorrugations.

2. An antenna as set forth in claim 1 wherein said slot corrugatedsurface further comprises corrugations protruding into said structure tocreate a reactive surface.

3. An antenna as set forth in claim 2 wherein said slot corrugations arevanishingly thin and wherein the energy reflected from the base of theslots is limited to the TEM mode.

4. An antenna as set forth in claim 1 wherein the reactance of saidcorrugated slot is given by i ll'i tan kn J g-l E 5. An antenna as setforth in claim 1 wherein the walls of 5 said corrugated slots arevanishingly thin.

1. An antenna including a pyramidal horn structure having a flare angleless than 90* for directing the electromagnetic energy in a givendirection and wherein energy is diffracted from certain of the surfacesof said structure, the improvement comprising means for controlling theillumination of said surfaces, said means including corrugated slotsformed in said surface perpendicular to the transverse E vector and theiris of said horn; wherein the depth of said slots is defined by and theslOt width and spacing is defined by with t much less than g, where g isthe slot width and t is the wall thickness of the corrugations.
 2. Anantenna as set forth in claim 1 wherein said slot corrugated surfacefurther comprises corrugations protruding into said structure to createa reactive surface.
 3. An antenna as set forth in claim 2 wherein saidslot corrugations are vanishingly thin and wherein the energy reflectedfrom the base of the slots is limited to the TEM mode.
 4. An antenna asset forth in claim 1 wherein the reactance of said corrugated slot isgiven by where g spacing between corrugations t wall thickness ofcorrugations d depth of corrugations and wherein
 5. An antenna as setforth in claim 1 wherein the walls of said corrugated slots arevanishingly thin.